Impact of the 1997-1998 El Nino and 1999 La Nina on nutrient supply in the Gulf of Alaska

Nutrient surveys of the Gulf of Alaska, from 1997 through 1999, show that coastal waters of British Columbia and southern Alaska experienced nitrate depletion each spring and summer. Through the 1997-1998 El Nino, waters with less than 1 muM NO3 covered 250,000 km(2) area greater than 1999. Silicate levels as low as 0,2 muM were observed in coastal waters, suggesting that diatom growth may have been nutrient limited both in 1998 and 1999. Detailed sampling off the southern coast of British Columbia revealed that 1998 nitrate levels were only half the average of that during the 1970s winter, were depleted I month earlier in spring and remained low throughout the summer. Satellite images show that, compared to 1997 and 1999, chlorophyll levels were much lower in the spring of 1998 throughout the coastal waters of the Gulf of Alaska. Conditions changed dramatically during the 1999 La Nina, with ocean-mixed layer depths increasing by 20 m in winter and 40 m in spring when compared to that during 1997-1998 El Nino. Winter nutrient levels increased and summer upwelling returned. Over the past several decades, a trend towards. greater stratification of coastal waters appears to be affecting the supply of nutrients to the mixed layer. The effects of stratification were especially obvious during the 1998 El Nino. (C) 2002 Elsevier Science Ltd. All rights reserved.